This invention pertains to a method and apparatus for applying resilient surfaces to be used for running tracks, tennis courts, playgrounds, jogging paths, ballfield warning tracks and other activity areas requiring resilience.
Many materials and methods of application have been used to produce all-weather surfaces for the aforementioned uses, including pre-manufactured and in situ types. These systems typically involve a mixture of rubber granules, which provide resilience and traction, and a liquid binder, which hardens or cures and thereby holds the rubber particles in a solid matrix.
Pre-manufactured products are expensive and difficult to install. Indeed, the installation of pre-manufactured products inevitably results in many seams or joints which can fail in outdoor use. Accordingly, most installations of all-weather surfaces have been of the in situ (formed on site) type. Currently, there are two basic methods of in situ installation, commonly referred to as "dry" and "wet" applied.
The wet application process involves mixing rubber particulate with liquid binder in a mixer at specific ratios and batch sizes (usually at a ratio, by weight, of 60% binder, 40% rubber). The resulting slurry is spread onto the area to be surfaced by hand or mechanical means. This application is usually done in multiple layers when using latex binder and in one mechanically paved layer when using urethane binders. With respect to the latter, the application is typically accomplished by means of a track driven paving machine with an oscillating oil heated screed. This installation method creates paving joints or seams approximately every eight feet, as well as transverse joints approximately every 100 to 200 feet. These joints are not only aesthetically objectionable, they also create weak links in the system which are subject to premature failure. To cover these joints and seams, multiple structural sprays are usually applied to paved base mat polyurethanes. However, this method is limited to a maximum particle size of approximately 2 mm, and requires a high ratio of binder. Attempts have been made to use this method with latex binders, however there is a tendency for the rubber to separate from the liquid and clog the hose. Moreover, even with latex binders the particle size is limited to a maximum of 2 mm. With rubber particle sizes larger than 2 mm, the velocity of the rubber exiting the tip of the spray nozzle was such that the rubber "bounced" when impacting the substrate, thereby separating the rubber from the liquid binder. Moreover, such small particle sizes means that surface thickness cannot be built up to typically required depths without intolerable cost in terms of time and materials, and without un acceptable loss of resilience and porosity in the resulting surface. Stated differently, one could spray apply a track surface to typical thickness (3/8 or 1/2") using rubber particles of less than 2 mm, but such a process would not be economically feasible. Hence, this method is inappropriate for surfaces with greater than 2 mm in depth because of the man-hours required for application of thicker surfaces. In addition, the rubber and binder are mixed in a hopper, and unless conveyed to the site of application promptly, may set prematurely either in the hopper, the hose, or the spray nozzle.
The structural spray coats are the standard method of adding color to this type of track surface. Structural spray coats consist of 0.5 to 1.5 mm EPDM rubber, polyurethane binder and color pigmentations. However, this surface traditionally shows premature shadowing (signs of "black through"). This shadowing occurs because the structural spray can only be applied in limited thicknesses or it will choke the surface creating problems of adhesion and delamination. Thus, these structural sprays are generally applied to a maximum of 2 mm thickness with 0.5 to 1.5 mm rubber.
Conventionally the most common method for applying latex tracks is called the "rake and spray" or "dry" method. This process involves simply evenly raking out a layer of dry rubber granules onto the track base and then spraying over the granulate with a latex binder. This process is repeated with successive layers of rubber until a desired thickness is reached.
Although this method provides a more affordable athletic surface than polyurethane surfaces, is seamless, and does not require heavy investment in equipment, it is flawed in several major respects:
1. The method depends totally on the applicator to assume that a uniform ratio of rubber to binder is maintained. This is extremely difficult, since it requires the applicators to spread rubber by hand at the same poundage per unit area at all points on the surface, and then requires that the sprayer of the liquid binder applies exactly the same volume of liquid to each unit area. Improper application renders surfaces installed by this method prone to inconsistent results which are manifested in weak or easily abraded areas of the surface.
2. The method relies on migration by gravity of the liquid binder through the rubber granules in order to accomplish encapsulation of all the rubber particles. The ability or extent of migration can vary significantly, however, with the poundage of rubber applied per unit area and with the sizing or gradation of the rubber. For example, the presence of more fines will greatly inhibit migration. Since the recycled ground rubbers used in a running track surface vary dramatically from load to load or even from bag to bag, migration and therefore encapsulation can vary greatly.
3. Latex binder does not have the same physical properties as polyurethane binder. Its resilience is more effected by temperature, which means the majority of latex track surfaces are very hard during track season (February through mid May). In addition, latex binder is very susceptible to moisture during curing, which causes unraveling and delamination.
This "rake and spray" method of the prior art tends to pack the rubber tightly, which means more rubber and therefore more binder is necessary for a given thickness.
The wet and dry methods of application each have the further disadvantage of being labor intensive and time consuming.
In view of the added difficulties associated with the dry application method, various attempts have been made to devise continuous wet application methods rather than batch for both latex and urethane binder systems.